The core production method known by the names “cold box method” or “Ashland method” has become very important in the foundry industry. In this method, two-component polyurethane systems are used to bind a refractory mold base material. The polyol component consists of a polyol having at least two OH groups per molecule, the isocyanate component consists of a polyisocyanate having at least two NCO groups per molecule. The binder system is cured with the aid of basic catalysts. Liquid bases can be mixed with the binder system before the molding process to bring the two components to reaction (U.S. Pat. No. 3,676,392). Another option is to pass gas-phase tertiary amines through the molding material-binder material mixture after the molding process (U.S. Pat. No. 3,409,579).
According to U.S. Pat. No. 3,676,392 and U.S. Pat. No. 3,409,579, phenolic resins that are obtained in the liquid phase by condensing phenol with aldehydes, preferably formaldehyde, at temperatures up to about 130° C. in the presence of catalytic quantities of metal ions are used as the polyols. The production of such phenolic resins is described in detail in U.S. Pat. No. 3,485,797. Substituted phenols, preferably o-cresol and p-nonylphenol, may be used as well as unsubstituted phenol (compare for example U.S. Pat. No. 4,590,229). According to EP 0177871 A2, phenolic resins that have been modified with aliphatic monoalcohols containing one to eight carbon atoms are used as a further reaction component. Alkoxylation is intended to lend the binder systems greater thermal stability.
The most frequently used solvents for the polyol component are mixtures of polar solvents with high boiling point (for example esters and ketones) and aromatic hydrocarbons with high boiling point. On the other hand, the polyisocyanates are preferably dissolved in aromatic hydrocarbons with high boiling point.
In EP 0771599 A1 and WO 00/25957 A1, formulations are described with which the use of fatty acid esters renders aromatic solvents entirely or at least largely unnecessary.
For a variety of reasons, it is desirable that such mold material mixtures have a very long processing time, that is to say the two components only react with one another when they come into contact with a catalyst. Even though many advances have been made in this regard since the invention of the “cold box method”, for example with the addition of phosphorus halides to the polyisocyanate component described in U.S. Pat. No. 4,540,724, even mold material mixtures produced with modern binder formulations have a limited useful life. This becomes evident as the molds and cores lose mechanical strength with the increasing age of the mold material mixtures, and after a certain time they fail to reach the value required to ensure safe handling and a good casting result. Japanese document JP 3794944 B2 suggests adding C6-C16-alkylbenzenes to prevent the mold material mixture from hardening prematurely.
One practical consequence of premature hardening is that relatively large quantities of mold material mixtures that have become unusable, after unforeseen interruption in production, for example, periodically have to be disposed of. The effort involved in cleaning machinery, supply and transport containers becomes particularly substantial when the mold material mixtures are no longer soft and flowable, but have (partially) hardened at the time of cleaning.
Users would therefore like to have mold material mixtures provided to them that not only have a very long processing time but also do not harden to form a solid mass after the processing time has elapsed. The mold material mixtures should still be soft and flowable then, so that the core machines are easy to clean.